The present invention relates to a bluish green pigment as coated with a metal oxide composite, which has a good color tone with excellent saturation and lightness and has high chemical safety, as well as to a method of preparing the same. The object of the present invention is to provide a bluish green pigment which is useful as a colorant for cosmetic materials, coating compositions, inks and plastics.
Green or bluish green flaky pigments, prepared by coating a blue or green pigment such as prussian blue or ultramarine pigment or chromium oxide on the surface of a flaky mica or a flaky mica as coated with a titanium oxide having an interference color of blue or green, as well as a colored flaky powder prepared by coating a metal oxide composite comprising oxides of titanium, cobalt and lithium on the surface of a flaky powder have been known.
Also known is a cobalt (II) ion-containing green pigment, specifically a spinel type titanium pigment such as cobalt titanate (CO.sub.2 TiO.sub.4 -CoTiO.sub.5), which is obtained by blending a cobalt oxide powder and a titanium oxide powder and then firing the resulting blend at a high temperature of 900.degree. C. or higher. Additionally, green pigments with improved color tone such as cobalt lithium titanate (Li.sub.2 CoTi.sub.3 O.sub.8) and a composition of CaTio.sub.3 --CoTiO.sub.3 --TiO.sub.2 are also known.
A bluish green pigment, having a chemical formula of XCoO(2-X)MgOTiO.sub.2 (X is from 0.8 to 1.0; and the main wavelength .lambda..sub.p is from 491 to 500 .mu.m) is known, and is prepared by reacting magnesium oxide and titanium oxide at a high temperature of about 1300.degree. C. to synthesize magnesium titanate (MgTiO.sub.3) having a spinel structure and doping cobalt oxide to the titanate by firing.
It is not possible to produce this pigment on a flaky substrate, however, such as in a process wherein a substrate of a flaky kaolin, talc, sericite or mica is coated with a cobalt (II) ion-containing spinel type titanium pigment, the flaky substrate is blended with cobalt oxide, magnesium oxide and titanium oxide and then fired at a high temperature. In this process, the flaky substrate and the oxides are solidified by firing, and a pigment is not obtained.
A green pigment prepared by coating a flaky powder with a metal oxide composite comprising oxides of titanium, cobalt and lithium is known (Japanese Patent Application Laid-Open No. 63-043963). However, lithium is an extremely scarce natural resource and is therefore high-priced as an industrial raw material. Additionally, the process of producing the pigment comprises complicated wet-process steps and dry-process steps where a titanium oxide hydrate and a cobalt oxide hydrate are precipitated on the surface of the flaky powder by a wet-process, then the thus precipitate-coated product is washed with water and dried to give a powder and the resulting powder is blended with a lithium compound and then fired at a high temperature. Consequently, the pigment is inevitably high-priced.
On the other hand, the above-mentioned pigment as prepared by coating a prussian blue on the surface of a titanium oxide-coated mica has some drawbacks that the coated prussian blue fades in an alkaline solution generating dicyan or is decomposed to fade under heat at a temperature of 200.degree. to 300.degree. C. Similarly, the pigment as coated with ultramarine has a problem of chemical stability, since it generates hydrogen sulfide gas in an acidic solution. The use of the other pigment as coated with chromium oxide is limited currently since chromium is said to have a problem of safety.